feat(editor): EmailBuilder canvas and lossless template serialization

[patoperpetua]

Summary

• Integrates the EmailBuilder.js visual canvas into @singleton-sd/post-kit-editor using @usewaypoint/email-builder@0.0.9 and the waypoint block editor stack (no global store; controlled React context).
• Adds loadTemplateSource() / serializeTemplateSource() for lossless, deterministic round-trips of template.json, metadata.json, and preview.json, preserving unknown forward-compatible fields.
• Wires EmailTemplateEditor to hold working TemplateSourceFiles in local state and update templateJson from canvas edits.

Test plan

• pnpm --filter @singleton-sd/post-kit-editor run test (15 tests: serialization round-trips, validation errors, compiler compatibility, canvas render)
• pnpm test (full workspace)
• CI Lint / test / build green on PR

Closes #40

Made with Cursor

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between a2dfa4e and 59553a0.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (30)

• packages/post-kit-editor/package.json
• packages/post-kit-editor/src/__fixtures__/minimal/metadata.json
• packages/post-kit-editor/src/__fixtures__/minimal/preview.json
• packages/post-kit-editor/src/__fixtures__/minimal/template.json
• packages/post-kit-editor/src/__fixtures__/nested-blocks/metadata.json
• packages/post-kit-editor/src/__fixtures__/nested-blocks/preview.json
• packages/post-kit-editor/src/__fixtures__/nested-blocks/template.json
• packages/post-kit-editor/src/__fixtures__/unknown-field/metadata.json
• packages/post-kit-editor/src/__fixtures__/unknown-field/preview.json
• packages/post-kit-editor/src/__fixtures__/unknown-field/template.json
• packages/post-kit-editor/src/canvas/EditorBlock.tsx
• packages/post-kit-editor/src/canvas/EmailBuilderCanvas.tsx
• packages/post-kit-editor/src/canvas/blocks/ColumnsContainerEditor.tsx
• packages/post-kit-editor/src/canvas/blocks/ColumnsContainerPropsSchema.ts
• packages/post-kit-editor/src/canvas/blocks/ContainerEditor.tsx
• packages/post-kit-editor/src/canvas/blocks/ContainerPropsSchema.ts
• packages/post-kit-editor/src/canvas/blocks/EmailLayoutEditor.tsx
• packages/post-kit-editor/src/canvas/blocks/EmailLayoutPropsSchema.ts
• packages/post-kit-editor/src/canvas/editor-context.tsx
• packages/post-kit-editor/src/canvas/editor-core.tsx
• packages/post-kit-editor/src/canvas/helpers/AddBlockMenu.tsx
• packages/post-kit-editor/src/canvas/helpers/EditorBlockWrapper.tsx
• packages/post-kit-editor/src/canvas/helpers/EditorChildrenIds.tsx
• packages/post-kit-editor/src/canvas/helpers/block-templates.ts
• packages/post-kit-editor/src/email-template-editor.tsx
• packages/post-kit-editor/src/index.spec.tsx
• packages/post-kit-editor/src/index.ts
• packages/post-kit-editor/src/serialization.spec.ts
• packages/post-kit-editor/src/serialization.ts
• packages/post-kit-editor/src/types.ts

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